1. Field of the Invention
This invention relates to a stainless steel that is an optimum material for members and components requiring corrosion resistance together with high strength and fatigue property, such as flat springs, coil springs, blade plates for Si single crystal wafer fabrication, particularly to an ultra-high strength metastable austenitic stainless steel having extremely high tensile strength, and a method of producing the same.
2. Background Art
When manufacturing members or components such as the foregoing from stainless steel, a martensitic stainless steel, work-hardened stainless steel or precipitation-hardened stainless steel has conventionally been used.
Martensitic stainless steels are produced by quenching from the high-temperature austenitic state to achieve hardening by martensite transformation. Examples include SUS410 and SUS420J2. High strength and toughness can be obtained by subjecting these steels to quench-anneal tempering treatment. When the product is extremely thin, however, it is deformed by the thermal strain during quenching. This makes it difficult to fabricate the product in the desired shape.
In the case of work-hardened stainless steels, a steel exhibiting austenite phase in the solution treatment state is thereafter cold-worked to generate strain-induced martensite phase for the purpose of obtaining high strength. Typical examples of these metastable austenitic stainless steels are SUS301 and SUS304. Their strength depends on the amount of cold-working and the amount of martensite. The problem of thermal strain during quenching mentioned above does not arise. Precise adjustment of strength solely by cold-working is, however, very difficult. When the cold-working rate is too high, anisotropy increases to degrade toughness.
Precipitation-hardened stainless steels are obtained by inclusion of an element with high precipitation hardness ability and age-hardening. SUS630, containing added Cu, and SUS631, containing added Al, are typical types. The former exhibits martensite single phase after solution treatment and is age-hardened from this state. The tensile strength achieved is only around 1400 N/mm2 at the greatest. The latter exhibits metastable austenite phase after solution treatment and is age-hardened after this phase has been partially converted to martensite phase by cold-working or other such preprocessing. The hardening is achieved by precipitation of the intermetallic compound Ni3Al and the tensile strength can be raised to around 1800 N/mm2 by positive generation of martensite phase.
Stainless steels utilizing such age-hardening also include ones developed to have higher strength than the foregoing conventional ones. Japanese Patent Application Laid-Open (KOKAI) No. 61-295356 (1986) and Laid-Open No. 4-202643 (1992), for instance, teach methods of subjecting metastable austenitic stainless steels added with Cu and Si in combination to an appropriate degree of cold-working followed by age-hardening. These methods provide high-strength steels of a tensile strength of around 2000 N/mm2. However, the age-hardening temperature range for obtaining high hardness by these methods is very narrow. Application to commercial production is therefore not easy.
In Japanese Patent Application Laid-Open No. 6-207250 (1994) (hereinafter ""250) and Laid-Open No. 7-300654 (1995) (hereinafter ""654), the present inventors later disclosed that a high-strength steel of a tensile strength of about 2000 N/mm2 and also excellent in toughness can be obtained by subjecting a metastable austenitic stainless steel added with Mo and Si in combination to an appropriate degree of cold-working and thereafter conducting age-hardening at a high temperature. Although this method requires strict control of the steel composition, this requirement can be fully met with today""s steelmaking techniques. Moreover, since the age-hardening temperature range is broad and age-hardening can be effected in a short time, the method is suitable for continuous production of steel strip.
The teachings of the aforesaid ""250 and ""654 can be said to have substantially established a production technology for high-strength stainless steel of 2000-N/mm2-class strength. Recently, however, an increasing need is being felt for stainless steel materials of still higher strength, mainly for use as spring material and in blade plates. To respond to this need, there should desirably be developed and supplied steel materials that can be reliably obtained with a tensile strength on not less than 2200 N/mm2.
On the other hand, 18 Ni maraging steel is known as an ultra-high strength metal material having tensile strength on the order of 2000-2400 N/mm2. For example, it is know that 18 Ni-9 Co-5 Mo-0.7 Ti-system maraging steel and 18 Ni-12.5 Co-4.2 Mo-1.6 Ti-system maraging steel achieve tensile strengths on the order of 2000 N/mm2 and 2400 N/mm2, respectively. These steels are also relatively good in toughness. They are, however, very high in cost because they contain large amounts of expensive elements like Ni, Co and Mo. Practical application of these steels as a material for inexpensive springs and the like is therefore impossible.
In view of the foregoing circumstances, the object of the present invention is to manufacture and provide an ultra-high strength metal material exhibiting a high tensile strength of not less than 2200 N/mm2 using metastable austenitic stainless steel as a material. Moreover, this invention is capable of providing not only steel strip obtained by aged on a continuous line but also steels that are aged by batch processing after processing into various components.
The inventors made various attempts to increase the tensile strength of the steels taught by ""250 and ""654 to the order of 2200 N/mm However, they were unable consistently obtain such high strength in these steels. Through further studies they learned that production of the steels taught by ""250 and ""654 at a strength exceeding 2000 N/mm2 involves a fundamental difficulty from the aspect of alloy design. They therefore concluded that development of a new steel having a different chemical composition was necessary. Pursuing this line of reasoning, they learned that, from the aspect of steel type, it is, as heretofore, advantageous to use a precipitation-hardened metastable austenitic stainless steel added with Mo and Cu and further that a high strength on the order of 2200 N/mm2 can be obtained by, differently from the conventional practice, adopting a composition system additionally containing Ti. They also learned that it is very preferable to conduct cold-working to generate strain induced martensite phase in the metallic texture so as to obtain a texture of 50-95 vol % of martensite+austenite before aging. This invention was accomplished based on this knowledge.
In a first aspect of the invention, the foregoing object is achieved by providing an ultra-high strength metastable austenitic stainless steel having a chemical composition comprising, in mass %, not more than 0.15% of C, more than 1.0 to 6.0% of Si, not more than 5.0% of Mn, 4.0-10.0% of Ni, 12.0-18.0% of Cr, not more than 3.5% of Cu, not more than 5.0% of Mo, not more than 0.02% of N, 0.1-0.5% of Ti, and the balance of Fe and unavoidable impurities, satisfying Si+Moxe2x89xa73.5%, having a value of Md(N) defined by equation (1) below of 20-140, exhibiting a cold worked multiphase texture composed of 50-95 vol % of martensite phase and the remainder substantially of austenite phase, and having Mo-system precipitates and Ti-system precipitates distributed in the martensite phase:
Md(N)=580-520C-2Si-16Mn-16Cr-23Ni-300N-26Cu-10Moxe2x80x83xe2x80x83(1).
By xe2x80x9csubstantially of austenite phasexe2x80x9d is meant that precipitates, intermetallic inclusions and small amount (roughly less than 1%) of xcex4 ferrite phase can be included. The presence of a cold worked texture can be determined from, for example, the fact that the austenite crystal grains are found to extend in the working direction when observed with an optical microscope. Typical Mo-system precipitates include Fe2Mo and Fe3Mo. Typical Ti-system precipitates include Ni16Ti6Si7 (G phase) and Ni3Ti. The presence of these precipitates can be determined by a microscopic observation method using an electron microscope, for example.
In a second aspect of the invention, an ultra-high strength metastable austenitic stainless steel according to the first aspect is provided wherein the steel further comprises at least one of not more than 0.5 mass % of V and not more than 0.5 mass % of Nb. In other words, the second aspect of the invention provides an ultra-high strength metastable austenitic stainless steel having a chemical composition comprising, in mass %, not more than 0.15% of C, more than 1.0 to 6.0% of Si, not more than 5.0% of Mn, 4.0-10.0% of Ni, 12.0-18.0% of Cr, not more than 3.5% of Cu, not more than 5.0% of Mo, not more than 0.02% of N, 0.1-0.5% of Ti, at least one of not more than 0.5% of V and not more than 0.5% of Nb, and the balance of Fe and unavoidable impurities, satisfying Si+Moxe2x89xa73.5%, having a value of Md(N) defined by equation (1) of 20-140, exhibiting a cold worked multiphase texture composed of 50-95 vol % of martensite phase and the remainder substantially of austenite phase, and having Mo-system precipitates and Ti-system precipitates distributed in the martensite phase.
In a third aspect of the invention, a steel according to the first or second aspect is provided wherein Cu content is 1.0-3.0 mass % and Mo content is 1.0-4.5 mass %.
In a fourth aspect of the invention, a steel according to any of the first to third aspects is provided wherein the steel is sheet steel or wire steel having a tensile strength of not less than 2200 N/mm2.
In a fifth aspect of the invention, a method of producing an ultra-high strength metastable austenitic stainless steel having a tensile strength of not less than 2200 N/mm2 is provided which comprises a step of solution-treating a steel having a chemical composition according to the first aspect of the invention, a step of cold-working the solution-treated steel to obtain a steel having a metallic texture composed of 50-95 vol % of martensite phase, and a step of aging the cold-worked steel in a temperature range of 300-600xc2x0 C. for 0.5-300 minutes. The xe2x80x9c50-95 vol % of martensite phasexe2x80x9d referred to here consists primarily of strain-induced martensite phase newly generated by the cold-working but also includes any cooling-induced martensite phase already present after the solution treatment. Portions other than the martensite phase are substantially austenite phase.
In a sixth aspect of the invention, the method according to the fifth aspect is applied to a steel further comprising at least one of not more than 0.5 mass % of V and not more than 0.5 mass % of Nb, i.e., a steel having a chemical composition according to the second aspect.
In a seventh aspect of the invention, the method according to the fifth or sixth aspect is applied to a steel wherein Cu content is 1.0-3.0 mass % and Mo content is 1.0-4.5 mass %.
In an eighth aspect of the invention, the method according to any of the fifth to seventh aspects is provided wherein the steel subjected to aging is a steel having a metallic texture composed of 50-95 vol % of martensite phase obtained by conducting the solution-treating step to attain a texture consisting of austenite single phase or a texture consisting primarily of austenite phase and containing not more than 30 vol % of cooling-induced martensite phase and thereafter cold-working the steel to generate strain-induced martensite phase.
In a ninth aspect of the invention, the method according to any of the fifth to eighth aspects is provided wherein the aging step is conducted batchwise for 10-300 minutes.